E-folding

In science, "e"-folding is the time interval in which an exponentially growing quantity increases by a factor of "e". This term is often used in theoretical physics, especially when cosmic inflation is investigated. Physicists often talk about the "e"-folding time scale that is determined by the proper time in which the length of a patch of space or spacetime increases by the factor "e" mentioned above.

In finance the logarithmic return or continuously compounded return, also known as force of interest, is the reciprocal of the e-folding time.

The term "e"-folding time is also sometimes used similarly in the case of exponential decay, to refer to the timescale for a quantity to decrease to 1/"e" of its previous value.

The process of evolving to equilibrium is often characterized by a time scale called the e-folding time, τ. This time is used for processes which evolve exponentially toward a final state (equilibrium). In other words if we examine an observable, X, associated with a system, (temperature or density for example) then after a time, τ, the initial difference between the initial value of the observable and the equilibrium value, ΔXi, will have decreased to ΔXi /e where the number e ~ 2.71828.